Photoemission and electron-energy-loss-spectroscopy study of ${\mathrm{C}}_{60}$ monolayers adsorbed on Cs-precovered Au(110) and of bulk distilled ${\mathrm{Cs}}_{\mathrm{x}}$${\mathrm{C}}_{60}$

The adsorption of ${\mathrm{C}}_{60}$ in submonolayer and monolayer coverages on Cs-precovered Au(110) has been investigated by photoemission and high-resolution electron-energy-loss spectroscopy. Complete fullerene monolayers display discrete molecular oxidation states of approximately -3, -4, and -6 in the presence of Cs. The oxidation states were determined by a comparison of monolayer electronic structure and vibrational mode frequencies with those of the bulk fullerides. There is no evidence for oxidation states of -2 and -5. Surface-specific measurements on bulk vacuum distilled alkali-fulleride samples showed a strong similarity between the properties of the single-layer films and the surface layers of the bulk material. Annealing experiments demonstrated that it was possible to produce a well-ordered, close-packed monolayer of ${\mathrm{C}}_{60}$ with an oxidation state of -3, which may be a useful precursor in producing a metastable fcc ${\mathrm{Cs}}_3$ ${\mathrm{C}}_{60}$ film by epitaxial growth. Bonding with the substrate stabilized this film with respect to bulk samples leading to desorption only at temperatures >900 K. Electron-energy-loss spectroscopy of submonolayer ${\mathrm{C}}_{60}$ in the presence of multilayers of Cs indicates that filling of the lowest unoccupied molecular orbital (LUMO)+1 states occurs. Completion of the monolayer results in extra charge occupying the LUMO band only. However, LUMO+1 occupation can also be brought about by sandwiching complete fullerene monolayers between Cs layers.